Re: Unlogged tables, persistent kind

Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> You're assuming that there are referential links *from* other tables
> to the table with damage. In which case you would be correct. But of
> course, if you needed that data for integrity you would never do that,
> so the problem is a nonexistent use case. The suggested mode is for
> Fact data, not reference tables.

So I suppose your notion of "fact data" includes no fields that are wide
enough to need toasting? Because as soon as you have any out-of-line
values, there's an equivalent of foreign keys behind the scenes, where
the user can't see it (until he gets "missing chunk number" or some such
error).

> The current assessment is that UNLOGGED tables are useful only for
> running a data cache. If the database crashes, then the table is
> truncated and you must refill the cache. If that is the case, then it
> must surely be better to have a cache that is already 99% full, than
> one which starts at empty. There is no damage or loss because parts of
> the cache were missing.

A cache that starts at 99% full of untrustworthy data is NOT better.

> Unlogged Tables are currently so volatile they are unusable for any
> other purpose. I want to see a table that is useful for low value
> data, such as sensor data.

Basically, you're being hopelessly optimistic both about the extent to
which a crash is likely to render data inconsistent, and our ability to
detect that inconsistency. It doesn't matter whether the data is "low
value", the difficulty of cleaning up remains the same. I don't want to
deal with trying to detect that, and I definitely don't want to dump the
problems onto users.

regards, tom lane